In order to reduce the power loss in a hydrodynamic torque converter, according to the prior art, a torque converter lock-up clutch is implemented with power transmissions, through which the turbine wheel of the converter can be connected to the impeller. When the torque converter lock-up clutch is engaged, a loss-free drive connection exists between the drive motor and the transmission of the vehicle.
With previously known applications for working machines, such as dump trucks, graders and mobile cranes, engagement and disengagement of the torque converter lock-up clutch is dependent on the rotational speed of the turbine taking into account the characteristic curve of the converter. For this, a disengaged torque converter lock-up clutch is actuated for engagement when the turbine rotational speed exceeds a threshold value at which the turbine torque is the same for both the engaged and disengaged torque converter lock-up clutch. According to the prior art, an engaged torque converter lock-up clutch is actuated for disengagement when the turbine rotational speed falls below a threshold level at which the turbine torque is the same for both the engaged and disengaged torque converter lock-up clutch. These shifting points are determined by the difference of the rotational speeds of the turbine and impeller.
With working machines such as wheeled loaders, forklifts, backhoe-loaders, telehandlers, etc., having hydraulically actuated lifting devices, which are driven in short working cycles and thereby resulting in a distribution of the performance of the internal combustion engine between the working hydraulics and the drive mechanism, the control of the torque converter lock-up clutch by means of the turbine rotational speed has proven to be disadvantageous. In particular, in the case of the load cycle, for example with a front-end or back-hoe loader, it is possible with this procedure that the engagement point of the torque converter lock-up clutch is reached precisely when the driver of the vehicle wants to slow down in front of a truck, thus resulting in the vehicle accelerating in a disadvantageous manner as a result of the engagement of the torque converter lock-up clutch. Furthermore, this procedure can also lead to there being too little power for the hydraulic mechanism, as a result of the low rotational speed of the engine, when the hydraulic mechanism is actuated while the torque converter lock-up clutch is engaged. The same disadvantage also occurs with fork-lifts when actuating the lifting device.
In a typical load cycle of a wheeled loader, the hydraulic mechanism is activated for lowering the shovel into the load and for raising the shovel when approaching a truck. A high rotational speed of the motor is necessary for this in order to provide the working hydraulic with enough power. With an engaged torque converter lock-up clutch, the motor is at a low rotational speed, wherein too little power is made available for the working hydraulic, in comparison to the operating conditions with a disengaged torque converter lock-up clutch.
It is known from the prior art that with wheeled loaders comprising a torque converter lock-up clutch, to prevent engaging the torque converter lock-up clutch in the lower gears.
Accordingly, an optimal usage of the torque converter lock-up clutch in working machines having short working cycles at low driving speeds in order to save fuel is not possible.
From DE 33 47 256 C2 a control device for a torque converter lock-up clutch of a hydrodynamic torque converter in the drive train of a towing vehicle, in particular a tractor for agricultural use having an internal combustion engine as a power source, a shifting clutch and a shifting transmission is known, wherein an auxiliary input shaft is provided, which rotates directly proportionally to the crankshaft of the driving motor, for driving working devices.
The torque converter lock-up clutch is automatically engaged, when a upper shifting rotational speed has been reached, and disengaged, at a lower shifting rotational speed, wherein the torque converter lock-up clutch is engaged and disengaged by means of a circuit provided for this purpose having inputs for the rotational speed of the turbine wheel of the torque converter and the rotational speed of the power-take-off shaft, which forms a control signal for the control valve of the torque converter, based on predefined values for the upper rotation speed and the lower rotational speed as well as the minimum working rotational speed and the maximum working rotational speed of the engaged power-take-off shaft.
As a result, a more effective power-take-off shaft, or a power-take off auxiliary drive mode, should be guaranteed with lower power loss of the torque converter.